A wide variety of dyes is employed in photographic materials. In addition to diverse dyes used to form images in color photographic elements, spectral sensitizing dyes are used to extend the sensitivity of silver halides, which are inherantly sensitive only to blue light, to other wavelengths of radiation. Among the dyes commonly employed for this purpose are the cyanines and merocyanines, which are discussed in T. H. James, editor, The Theory of the Photographic Process, 4th Edition, Macmillan, New York, 1977, Chapter 8, and in F. M. Hamer, Cyanine Dyes and Related Compounds, Wiley, New York, 1964.
Dyes are also used in color photographic materials as filters, typically located in overcoats, interlayers or undercoats, to absorb incident radiation and improve image sharpness. Intergrain absorber dyes may also be added directly to a spectrally sensitized silver halide emulsion to absorb light and thereby modify the sensitivity of the emulsion. In addition to the previously mentioned cyanine and merocyanines, various oxonol and arylidene dyes are frequently utilized for these purposes. A discussion of arylidene dyes can be found in K. Venkataraman, The Chemistry of Synthetic Dyes, Academic Press, New York, 1970, Volume III.
Historically, the oxonol class of dyes have been particularly useful in photographic systems for light filtration only. For that reason, a very large number of oxonol dye structural modifications have been described in the patent literature. For example a wide variety of ketomethylenes, or acidic methylene groups, may compose the end groups of the chromophore. Such groups may be carbocyclic, heterocyclic, or a fused ring system. Such ring systems are well known in the literature and are sometimes referred to as ketomethylenes. For example, those listed in Hamer, Cyanine Dyes and Related Compounds, pages 469-494 and 595-604, which include the ketomethylene groups benzoylacetonitrile, 2-pyrazolin-5-one, pyrazolindione, chromandione, cyclohexanedione, dioxanedione, furanone, isoxazolinone, and pyrandione among others.
In addition, hydroxypyridone oxonol dyes have been described in GB 1,278,621, and pyrazolopyridine oxonol dyes have been described in EP-A-0 295 698. The search for new bathochromic dyes has led to some novel oxonol dye dye structures. For example, tricyanopropene dyes are described in U.S. Pat. No. 5,213,956, while novel imines dyes are described in U.S. Pat. No. 5,260,179.
Oxonol dyes may typically be monomethine, trimethine, pentamethine, or heptamethine oxonol dyes. The methine (CH) units are often unsubstituted although some substituted methine (L=CR) units have also been described. For example, Hall, Burrows, and Kirk report in European Patent Application 397,435 that when G in the formula therein represents an oxygen atom and m is equal to 3 some of the groups L.sup.1 to L.sup.3 represent the elements of a 5, 6, or 7 membered carbocyclic, heterocyclic or fused ring system. U.S. Pat. No. 4,042,397 and U.S. Pat. No. 3,653,905 describe dyes with a substituted barbituric acid group in which the methines may be unsbustituted or substituted with, for example, alkyl, aralkyl, aryl or carboxyl. U.S. Pat. No. 5,035,977 describes oxonol type dyes with any of a large variety of possible methine substituents.
Given the usefulness of oxonol type dyes, it would be desirable to provide new classes of oxonol dyes which exhibit good characteristics as a filter dye in photographic elements and which may be readily prepared.